A CMOS oversampling D/A converter with a current-mode semi-digital reconstruction filter

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Abstract
The architecture of an oversampling D/A (digital-to-analog) converter is shown. A 16-b digital word at the Nyquist sampling rate of 44.1 kHz is unsampled to 7.58 MHz by a digital interpolator. These 16-b data are then quantized into a one-bit output by a digital second-order sigma-delta modulation noise shaper. Instead of converting the one-bit output of the noise shaper into a two-level analog signal, the one-bit code is processed by a semi-digital reconstruction filter that provides both analog reconstruction filtering and D/A conversion. A prototype D/A converter consisting of a second-order digital noise shaper and a 128-tap semi-digital reconstruction filter was fabricated in 1- mu m CMOS in an active area of about 2.5 mm/sup 2/. The simulated frequency spectrum of the analog output is shown for a 4.74-kHz sinusoidal input 10 dB below overload with an oversampling ratio of 172. The reconstruction filter reduces by 61 dB the total out-of-band power between 22.1 kHz and 3.79 MHz. Also shown is the measured signal-to-(noise+distortion) of the converter as a function of input signal level for a baseband of 22.1 kHz. Measurements indicate a dynamic range of 95 dB with a peak signal-to-(noise+distortion) ratio of 85 dB.

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